THE EVIDENCE OF THE THYROID GLAND 211 
is there because it is the remnant of the great glandular mass which 
so closely surrounds the brain and alimentary canal in animals such 
as Limulus. In my paper in the Quarterly Journal of Microscopical 
Science, in which I was comparing the tube of the vertebrate nervous 
system with the alimentary canal of the invertebrate, I spoke of this 
tissue as being the remnant of the invertebrate liver. At the same 
time the whole point of my argument was that the glandular ‘material 
surrounding the brain of Limulus was made up of two glands—liver 
and generative gland—so that this tissue might be the remnant of 
either one or the other, or both. All I desired, at that time, was 
to point out the glandular appearance of this so-called packing tissue, 
which surrounded the brain-region of Ammoceetes, in connection with 
the fact that the brain and alimentary canal of Limulus were closely 
surrounded with a glandular mass composed partly of liver, partly of 
the generative gland. At present, I think these large cells found 
round the brain in Ammoccetes are much more likely to be the 
remnant of the generative gland than of the liver; the size of the 
cells and their arrangement recalls Owen's picture of the generative 
gland in Limulus, and seeing how important all generative glands 
are in their capacity of internal secreting glands, apart entirely from 
the extrusion of the ripe generative products, and how unimportant 
is an hepato-pancreas when the alimentary canal is closed, it is much 
more likely that of the two glands the former would persist longer 
than the latter. It may be that all that is left of the old hepato- 
pancreas consists of the pigment so markedly found in between these 
cells, especially at the place where the old liver-duct reaches the 
surface of the brain; just as the only remnant of the two pineal eyes 
in the higher vertebrates is the remains of the pigment, known as 
brain-sand, which still exists in the pineal gland of even the highest 
vertebrate. This, however, is a mere speculation of no importance, 
What is important is the recognition of this tissue round the brain 
as the remnant of the glandular mass round the brain of animals such 
as Limulus. Still further confirmation of the truth of this comparison 
will be given when the origin of the auditory organ comes up for 
discussion. 
I conclude, therefore, from the evidence of Ammoccetes, that the 
generative glands in the ancestral form were situated largely in the 
cephalic region, and suggest that the course and direction of the ciliated 
pseudo-branchial grooves on each side indicate the direction of the 
